Internet Engineering Task Force Sally Floyd
INTERNET-DRAFT ICSI
draft-ietf-tsvwg-highspeed-00.txt 21 July 2003
Expires: January 2004
HighSpeed TCP for Large Congestion Windows
Status of this Memo
The proposals in this document are experimental. We believe they
are safe for deployment in the current Internet, but they do not
represent a consensus that this is the best method for high-speed
congestion control. In particular, we note that alternative
experimental proposals are likely to be forthcoming, and it is not
well understood how the proposals in this document will interact
with such alternative proposals.
Status of this Document
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet- Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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Abstract
This document proposes HighSpeed TCP, a modification to TCP's
congestion control mechanism for use with TCP connections with
large congestion windows. The congestion control mechanisms
of the current Standard TCP constrains the congestion windows
that can be achieved by TCP in realistic environments. For
example, for a Standard TCP connection with 1500-byte packets
and a 100 ms round-trip time, achieving a steady-state
throughput of 10 Gbps would require an average congestion
window of 83,333 segments, and a packet drop rate of at most
one congestion event every 5,000,000,000 packets (or
equivalently, at most one congestion event every 1 2/3 hours).
This is widely acknowledged as an unrealistic constraint. To
address this limitation of TCP, this document proposes
HighSpeed TCP, and solicits experimentation and feedback from
the wider community.
TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:
Changes from draft-floyd-tcp-highspeed-03.txt:
Added the section on "Status of this Memo".
Added a paragraph to the end of the section on "Deployment
issues of HighSpeed TCP" about possible interactions between
HighSpeed TCP and other alternative experimental proposals.
Changes from draft-floyd-tcp-highspeed-02.txt:
* Added a section on "Deployment issues."
* Added a short section on "Implementation issues."
* Added a section on "Limiting burstiness on short time
scales".
* Added to the discussion on convergence times.
* Clarified that "log" is "log base 10".
* Clarified that W = Low_window and W_1 = High_window, in the
equation for b(w).
Changes from draft-floyd-tcp-highspeed-01.txt:
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* Added a section on "Tradeoffs for Choosing Congestion
Control Parameters".
* Added mention of Scalable TCP from Tom Kelly.
Changes from draft-floyd-tcp-highspeed-00.txt:
* Added a discussion on related work about changing the PMTU.
* Added a discussion of an alternate, linear response
function.
* Added a discussion of the TCP window scale option.
* Added a discussion of HighSpeed TCP as roughly emulating the
congestion control response of N parallel TCP connections.
* Added a discussion of the time to converge to fairness.
* Expanded the Introduction.
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This document proposes HighSpeed TCP, a modification to TCP's
congestion control mechanism for use with TCP connections with large
congestion windows. In a steady-state environment, with a packet
loss rate p, the current Standard TCP's average congestion window is
roughly 1.2/sqrt(p) segments. This places a serious constraint on
the congestion windows that can be achieved by TCP in realistic
environments. For example, for a Standard TCP connection with
1500-byte packets and a 100 ms round-trip time, achieving a steady-
state throughput of 10 Gbps would require an average congestion
window of 83,333 segments, and a packet drop rate of at most one
congestion event every 5,000,000,000 packets (or equivalently, at
most one congestion event every 1 2/3 hours). The average packet
drop rate of at most 2*10^(-10) needed for full link utilization in
this environment corresponds to a bit error rate of at most
2*10^(-14), and this is an unrealistic requirement for current
networks.
To address this fundamental limitation of TCP and of the TCP
response function (the function mapping the steady-state packet drop
rate to TCP's average sending rate in packets per round-trip time),
this document describes a modified TCP response function for regimes
with higher congestion windows. This document also solicits
experimentation and feedback on HighSpeed TCP from the wider
community.
Because HighSpeed TCP's modified response function would only take
effect with higher congestion windows, HighSpeed TCP does not modify
TCP behavior in environments with mild to heavy congestion, and
therefore does not introduce any new dangers of congestion collapse.
However, if relative fairness between HighSpeed TCP connections is
to be preserved, then in our view any modification to the TCP
response function should be addressed in the IETF, rather than made
as ad hoc decisions by individual implementors or TCP senders.
Modifications to the TCP response function would also have
implications for transport protocols that use TFRC and other forms
of equation-based congestion control, as these congestion control
mechanisms directly use the TCP response function [RFC3448].
This proposal for HighSpeed TCP focuses specifically on a proposed
change to the TCP response function, and its implications for TCP.
This document does not address what we view as a separate
fundamental issue, of the mechanisms required to enable best-effort
connections to *start* with large initial windows. In our view,
while HighSpeed TCP proposes a somewhat fundamental change to the
TCP response function, at the same time it is a relatively simple
change to implement in a single TCP sender, and presents no dangers
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in terms of congestion collapse. In contrast, in our view, the
problem of enabling connections to *start* with large initial
windows is inherently more risky and structurally more difficult,
requiring some form of explicit feedback from all of the routers
along the path. This is another reason why we would propose
addressing the problem of starting with large initial windows
separately, and on a separate timetable, from the problem of
modifying the TCP response function.
2. The Problem Description.
This section describes the number of round-trip times between
congestion events required for a Standard TCP flow to achieve an
average throughput of B bps, given packets of D bytes and a round-
trip time of R seconds. A congestion event refers to a window of
data with one or more dropped or ECN-marked packets (where ECN
stands for Explicit Congestion Notification).
From Appendix A, achieving an average TCP throughput of B bps
requires a loss event at most every BR/(12D) round-trip times. This
is illustrated in Table 1, for R = 0.1 seconds and D = 1500 bytes.
The table also gives the average congestion window W of BR/(8D), and
the steady-state packet drop rate P of 1.5/W^2.
TCP Throughput (Mbps) RTTs Between Losses W P
--------------------- ------------------- ---- -----
1 5.5 8.3 0.02
10 55.5 83.3 0.0002
100 555.5 833.3 0.000002
1000 5555.5 8333.3 0.00000002
10000 55555.5 83333.3 0.0000000002
Table 1: RTTs Between Congestion Events for Standard TCP, for
1500-Byte Packets and a Round-Trip Time of 0.1 Seconds.
This document proposes HighSpeed TCP, a minimal modification to
TCP's increase and decrease parameters, for TCP connections with
larger congestion windows, to allow TCP to achieve high throughput
with more realistic requirements for the steady-state packet drop
rate. Equivalently, HighSpeed TCP has more realistic requirements
for the number of round-trip times between loss events.
3. Design Guidelines.
Our proposal for HighSpeed TCP is motivated by the following
requirements:
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* Achieve high per-connection throughput without requiring
unrealistically low packet loss rates.
* Reach high throughput reasonably quickly when in slow-start.
* Reach high throughput without overly long delays when recovering
from multiple retransmit timeouts, or when ramping-up from a period
with small congestion windows.
* No additional feedback or support required from routers:
For example, the goal is for acceptable performance in both ECN-
capable and non-ECN-capable environments, and with Drop-Tail as well
as with Active Queue Management such as RED in the routers.
* No additional feedback required from TCP receivers.
* TCP-compatible performance in environments with moderate or high
congestion:
Equivalently, the requirement is that there be no additional load on
the network (in terms of increased packet drop rates) in
environments with moderate or high congestion.
* Performance at least as good as Standard TCP in environments with
moderate or high congestion.
* Acceptable transient performance, in terms of increases in the
congestion window in one round-trip time, responses to severe
congestion, and convergence times to fairness.
Currently, users wishing to achieve throughputs of 1 Gbps or more
typically open up multiple TCP connections in parallel, or use
MulTCP [CO98,GRK99], which behaves roughly like the aggregate of N
virtual TCP connections. While this approach suffices for the
occasional user on well-provisioned links, it leaves the parameter N
to be determined by the user, and results in more aggressive
performance and higher steady-state packet drop rates if used in
environments with periods of moderate or high congestion. We
believe that a new approach is needed that offers more flexibility,
more effectively scales to a wide range of available bandwidths, and
competes more fairly with Standard TCP in congested environments.
4. Non-Goals.
The following are explicitly *not* goals of our work:
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* Non-goal: TCP-compatible performance in environments with very low
packet drop rates.
We note that our proposal does not require, or deliver, TCP-
compatible performance in environments with very low packet drop
rates, e.g., with packet loss rates of 10^-5 or 10^-6. As we
discuss later in this document, we assume that Standard TCP is
unable to make effective use of the available bandwidth in
environments with loss rates of 10^-6 in any case, so that it is
acceptable and appropriate for HighSpeed TCP to perform more
aggressively than Standard TCP is such an environment.
* Non-goal: Ramping-up more quickly than allowed by slow-start.
It is our belief that ramping-up more quickly than allowed by slow-
start would necessitate more explicit feedback from routers along
the path. The proposal for HighSpeed TCP is focused on changes to
TCP that could be effectively deployed in the current Internet
environment.
* Non-goal: Avoiding oscillations in environments with only one-way,
long-lived flows all with the same round-trip times.
While we agree that attention to oscillatory behavior is useful,
avoiding oscillations in aggregate throughput has not been our
primary consideration, particularly for simplified environments
limited to one-way, long-lived flows all with the same, large round-
trip times. Our assessment is that some oscillatory behavior in
these extreme environments is an acceptable price to pay for the
other benefits of HighSpeed TCP.
5. Modifying the TCP Response Function.
The TCP response function, w = 1.2/sqrt(p), gives TCP's average
congestion window w in MSS-sized segments, as a function of the
steady-state packet drop rate p [FF98]. This TCP response function
is a direct consequence of TCP's Additive Increase Multiplicative
Decrease (AIMD) mechanisms of increasing the congestion window by
roughly one segment per round-trip time in the absence of
congestion, and halving the congestion window in response to a
round-trip time with a congestion event. This response function for
Standard TCP is reflected in the table below. In this proposal we
restrict our attention to TCP performance in environments with
packet loss rates of at most 10^-2, and so we can ignore the more
complex response functions that are required to model TCP
performance in more congested environments with retransmit timeouts.
From Appendix A, an average congestion window of W corresponds to an
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average of 2/3 W round-trip times between loss events for Standard
TCP (with the congestion window varying from 2/3 W to 4/3 W).
Packet Drop Rate P Congestion Window W RTTs Between Losses
------------------ ------------------- -------------------
10^-2 12 8
10^-3 38 25
10^-4 120 80
10^-5 379 252
10^-6 1200 800
10^-7 3795 2530
10^-8 12000 8000
10^-9 37948 25298
10^-10 120000 80000
Table 2: TCP Response Function for Standard TCP. The average
congestion window W in MSS-sized segments is given as a function of
the packet drop rate P.
To specify a modified response function for HighSpeed TCP, we use
three parameters, Low_Window, High_Window, and High_P. To ensure
TCP compatibility, the HighSpeed response function uses the same
response function as Standard TCP when the current congestion window
is at most Low_Window, and uses the HighSpeed response function when
the current congestion window is greater than Low_Window. In this
document we set Low_Window to 38 MSS-sized segments, corresponding
to a packet drop rate of 10^-3 for TCP.
To specify the upper end of the HighSpeed response function, we
specify the packet drop rate needed in the HighSpeed response
function to achieve an average congestion window of 83000 segments.
This is roughly the window needed to sustain 10 Gbps throughput, for
a TCP connection with the default packet size and round-trip time
used earlier in this document. For High_Window set to 83000, we
specify High_P of 10^-7; that is, with HighSpeed TCP a packet drop
rate of 10^-7 allows the HighSpeed TCP connection to achieve an
average congestion window of 83000 segments. We believe that this
loss rate sets an achievable target for high-speed environments,
while still allowing acceptable fairness for the HighSpeed response
function when competing with Standard TCP in environments with
packet drop rates of 10^-4 or 10^5.
For simplicity, for the HighSpeed response function we maintain the
property that the response function gives a straight line on a log-
log scale (as does the response function for Standard TCP, for low
to moderate congestion). This results in the following response
function, for values of the average congestion window W greater than
Low_Window:
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W = (p/Low_P)^S Low_Window,
for Low_P the packet drop rate corresponding to Low_Window, and for
S as following constant [FRS02]:
S = (log High_Window - log Low_Window)/(log High_P - log Low_P).
(In this paper, "log x" refers to the log base 10.) For example,
for Low_Window set to 38, we have Low_P of 10^-3 (for compatibility
with Standard TCP). Thus, for High_Window set to 83000 and High_P
set to 10^-7, we get the following response function:
W = 0.12/p^0.835. (1)
This HighSpeed response function is illustrated in Table 3 below.
For HighSpeed TCP, the number of round-trip times between losses,
1/(pW), equals 12.7 W^0.2, for W > 38 segments.
Packet Drop Rate P Congestion Window W RTTs Between Losses
------------------ ------------------- -------------------
10^-2 12 8
10^-3 38 25
10^-4 263 38
10^-5 1795 57
10^-6 12279 83
10^-7 83981 123
10^-8 574356 180
10^-9 3928088 264
10^-10 26864653 388
Table 3: TCP Response Function for HighSpeed TCP. The average
congestion window W in MSS-sized segments is given as a function of
the packet drop rate P.
We believe that the problem of backward compatibility with Standard
TCP requires a response function that is quite close to that of
Standard TCP for loss rates of 10^-1, 10^-2, or 10^-3. We believe,
however, that such stringent TCP-compatibility is not required for
smaller loss rates, and that an appropriate response function is one
that gives a plausible packet drop rate for a connection throughput
of 10 Gbps. This also gives a slowly increasing number of round-
trip times between loss events as a function of a decreasing packet
drop rate.
Another way to look at the HighSpeed response function is to
consider that HighSpeed TCP is roughly emulating the congestion
control response of N parallel TCP connections, where N is initially
one, and where N increases as a function of the HighSpeed TCP's
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congestion window. Thus for the HighSpeed response function in
Equation (1) above, the response function can be viewed as
equivalent to that of N(W) parallel TCP connections, where N(W)
varies as a function of the congestion window W. Recall that for a
single standard TCP connection, the average congestion window equals
1.2/sqrt(p). For N parallel TCP connections, the aggregate
congestion window for the N connections equals N*1.2/sqrt(p). From
the HighSpeed response function in Equation (1) and the relationship
above, we can derive the following:
N(W) = 0.23*W^(0.4)
for N(W) the number of parallel TCP connections emulated by the
HighSpeed TCP response function, and for N(W) >= 1. This is shown
in Table 4 below.
Congestion Window W Number N(W) of Parallel TCPs
------------------- -------------------------
1 1
10 1
100 1.4
1,000 3.6
10,000 9.2
100,000 23.0
Table 4: Number N(W) of parallel TCP connections roughly emulated by
the HighSpeed TCP response function.
We do not in this document attempt to seriously evaluate the
HighSpeed response function for congestion windows greater than
100,000 packets. We believe that we will learn more about the
requirements for sustaining the throughput of best-effort
connections in that range as we gain more experience with HighSpeed
TCP with congestion windows of thousands and tens of thousands of
packets. There also might be limitations to the per-connection
throughput that can be realistically achieved for best-effort
traffic, in terms of congestion window of hundreds of thousands of
packets or more, in the absence of additional support or feedback
from the routers along the path.
6. Fairness Implications of the HighSpeed Response Function.
The Standard and Highspeed Response Functions can be used directly
to infer the relative fairness between flows using the two response
functions. For example, given a packet drop rate P, assume that
Standard TCP has an average congestion window of W_Standard, and
HighSpeed TCP has a higher average congestion window of W_HighSpeed.
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In this case, a single HighSpeed TCP connection is receiving
W_HighSpeed/W_Standard times the throughput of a single Standard TCP
connection competing in the same environment.
This relative fairness is illustrated below in Table 5, for the
parameters used for the Highspeed response function in the section
above. The second column gives the relative fairness, for the
steady-state packet drop rate specified in the first column. To
help calibrate, the third column gives the aggregate average
congestion window for the two TCP connections, and the fourth column
gives the bandwidth that would be needed by the two connections to
achieve that aggregate window and packet drop rate, given 100 ms
round-trip times and 1500-byte packets.
Packet Drop Rate P Fairness Aggregate Window Bandwidth
------------------ -------- ---------------- ---------
10^-2 1.0 24 2.8 Mbps
10^-3 1.0 76 9.1 Mbps
10^-4 2.2 383 45.9 Mbps
10^-5 4.7 2174 260.8 Mbps
10^-6 10.2 13479 1.6 Gbps
10^-7 22.1 87776 10.5 Gbps
Table 5: Relative Fairness between the HighSpeed and Standard
Response Functions.
Thus, for packet drop rates of 10^-4, a flow with the HighSpeed
response function can expect to receive 2.2 times the throughput of
a flow using the Standard response function, given the same round-
trip times and packet sizes. With packet drop rates of 10^-6 (or
10^-7), the unfairness is more severe, and we have entered the
regime where a Standard TCP connection requires at most one
congestion event every 800 (or 2530) round-trip times in order to
make use of the available bandwidth. Our judgement would be that
there are not a lot of TCP connections effectively operating in this
regime today, with congestion windows of thousands of packets, and
that therefore the benefits of the HighSpeed response function would
outweigh the unfairness that would be experienced by Standard TCP in
this regime. However, one purpose of this document is to solicit
feedback on this issue. The parameter Low_Window determines
directly the point of divergence between the Standard and HighSpeed
Response Functions.
The third column of Table 5, the Aggregate Window, gives the
aggregate congestion window of the two competing TCP connections,
with HighSpeed and Standard TCP, given the packet drop rate
specified in the first column. From Table 5, a HighSpeed TCP
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connection would receive ten times the bandwidth of a Standard TCP
in an environment with a packet drop rate of 10^-6. This would
occur when the two flows sharing a single pipe achieved an aggregate
window of 13479 packets. Given a round-trip time of 100 ms and a
packet size of 1500 bytes, this would occur with an available
bandwidth for the two competing flows of 1.6 Gbps.
Next we consider the time that it takes a standard or HighSpeed TCP
flow to converge to fairness against a pre-existing HighSpeed TCP
flow. The worst case for convergence to fairness occurs when a new
flow is starting up, competing against a high-bandwidth existing
flow, and the new flow suffers a packet drop and exits slow-start
while its window is still small. In the worst case, consider that
the new flow has entered the congestion avoidance phase while its
window is only one packet. A standard TCP flow in congestion
avoidance increases its window by at most one packet per round-trip
time, and after N round-trip times has only achieved a window of N
packets (when starting with a window of 1 in the first round-trip
time). In contrast, a HighSpeed TCP flows increases much faster
than a standard TCP flow while in the congestion avoidance phase,
and we can expect its convergence to fairness to be much better.
This is shown in Table 6 below. The script used to generate this
table is given in Appendix C.
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RTT HS_Window Standard_TCP_Window
--- --------- -------------------
100 131 100
200 475 200
300 1131 300
400 2160 400
500 3601 500
600 5477 600
700 7799 700
800 10567 800
900 13774 900
1000 17409 1000
1100 21455 1100
1200 25893 1200
1300 30701 1300
1400 35856 1400
1500 41336 1500
1600 47115 1600
1700 53170 1700
1800 59477 1800
1900 66013 1900
2000 72754 2000
Table 6: For a HighSpeed and a Standard TCP connection, the
congestion window during congestion avoidance phase (starting with a
congestion window of 1 packet during RTT 1.
The classic paper on relative fairness is from Chiu and Jain [CJ89].
This paper shows that AIMD (Additive Increase Multiplicative
Decrease) converges to fairness in an environment with synchronized
congestion events. From [CJ89], it is easy to see that MIMD and
AIAD do not converge to fairness in this environment. However, the
results of [CJ89] do not apply to an asynchronous environment such
as that of the current Internet, where the frequency of congestion
feedback can be different for different flows. For example, it has
been shown that MIMD converges to fair states in a model with
proportional instead of synchronous feedback in terms of packet
drops [GV02]. Thus, we are not concerned about abandoning a strict
model of AIMD for HighSpeed TCP.
7. Translating the HighSpeed Response Function into Congestion Control
Parameters.
For equation-based congestion control such as TFRC, the HighSpeed
Response Function above could be used directly by the TFRC
congestion control mechanism. However, for TCP the HighSpeed
response function has to be translated into additive increase and
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multiplicative decrease parameters. The HighSpeed response function
cannot be achieved by TCP with an additive increase of one segment
per round-trip time and a multiplicative decrease of halving the
current congestion window; HighSpeed TCP will have to modify either
the increase or the decrease parameter, or both. We have concluded
that HighSpeed TCP is most likely to achieve an acceptable
compromise between moderate increases and timely decreases by
modifying both the increase and the decrease parameter.
That is, for HighSpeed TCP let the congestion window increase by
a(w) segments per round-trip time in the absence of congestion, and
let the congestion window decrease to w(1-b(w)) segments in response
to a round-trip time with one or more loss events. Thus, in
response to a single acknowledgement HighSpeed TCP increases its
congestion window in segments as follows:
w <- w + a(w)/w.
In response to a congestion event, HighSpeed TCP decreases as
follows:
w <- (1-b(w))w.
For Standard TCP, a(w) = 1 and b(w) = 1/2, regardless of the value
of w. HighSpeed TCP uses the same values of a(w) and b(w) for w <=
Low_Window. This section specifies a(w) and b(w) for HighSpeed TCP
for larger values of w.
For w = High_Window, we have specified a loss rate of High_P. From
[FRS02], or from elementary calculations, this requires the
following relationship between a(w) and b(w) for w = High_Window:
a(w) = High_Window^2 * High_P * 2 * b(w)/(2-b(w). (2)
We use the parameter High_Decrease to specify the decrease parameter
b(w) for w = High_Window, and use Equation (2) to derive the
increase parameter a(w) for w = High_Window. Along with High_P =
10^-7 and High_Window = 83000, for example, we specify High_Decrease
= 0.1, specifying that b(83000) = 0.1, giving a decrease of 10%
after a congestion event. Equation (2) then gives a(83000) = 72,
for an increase of 72 segments, or just under 0.1%, within a round-
trip time, for w = 83000.
This moderate decrease strikes us as acceptable, particularly when
coupled with the role of TCP's ACK-clocking in limiting the sending
rate in response to more severe congestion [BBFS01]. A more severe
decrease would require a more aggressive increase in the congestion
window for a round-trip time without congestion. In particular, a
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decrease factor High_Decrease of 0.5, as in Standard TCP, would
require an increase of 459 segments per round-trip time when w =
83000.
Given decrease parameters of b(w) = 1/2 for w = Low_Window, and b(w)
= High_Decrease for w = High_Window, we are left to specify the
value of b(w) for other values of w > Low_Window. From [FRS02], we
let b(w) vary linearly as the log of w, as follows:
b(w) = (High_Decrease - 0.5) (log(w)-log(W)) / (log(W_1)-log(W)) +
0.5,
for W = Low_window and W_1 = High_window. The increase parameter
a(w) can then be computed as follows:
a(w) = w^2 * p(w) * 2 * b(w)/(2-b(w)),
for p(w) the packet drop rate for congestion window w. From
inverting Equation (1), we get p(w) as follows:
p(w) = 0.078/w^1.2.
We assume that experimental implementations of HighSpeed TCP for
further investigation will use a pre-computed look-up table for
finding a(w) and b(w). For example, the implementation from Tom
Dunigan adjusts the a(w) and b(w) parameters every 0.1 seconds. In
the appendix we give such a table for our default values of
Low_Window = 38, High_Window = 83,000, High_P = 10^-7, and
High_Decrease = 0.1. These are also the default values in the NS
simulator; example simulations in NS can be run with the command
"./test-all-tcpHighspeed" in the directory tcl/test.
8. An alternate, linear response functions.
In this section we explore an alternate, linear response function
for HighSpeed TCP that has been proposed by a number of other
people, in particular by Glenn Vinnicombe and Tom Kelly. Similarly,
it has been suggested by others that a less "ad-hoc" guideline for a
response function for HighSpeed TCP would be to specify a constant
value for the number of round-trip times between congestion events.
Assume that we keep the value of Low_Window as 38 MSS-sized
segments, indicating when the HighSpeed response function diverges
from the current TCP response function, but that we modify the
High_Window and High_P parameters that specify the upper range of
the HighSpeed response function. In particular, consider the
response function given by High_Window = 380,000 and High_P = 10^-7,
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with Low_Window = 38 and Low_P = 10^-3 as before.
Using the equations in Section 5, this would give the following
Linear response function, for w > Low_Window:
W = 0.038/p.
This Linear HighSpeed response function is illustrated in Table 7
below. For HighSpeed TCP, the number of round-trip times between
losses, 1/(pW), equals 1/0.38, or equivalently, 26, for W > 38
segments.
Packet Drop Rate P Congestion Window W RTTs Between Losses
------------------ ------------------- -------------------
10^-2 12 8
10^-3 38 26
10^-4 380 26
10^-5 3800 26
10^-6 38000 26
10^-7 380000 26
10^-8 3800000 26
10^-9 38000000 26
10^-10 380000000 26
Table 7: An Alternate, Linear TCP Response Function for HighSpeed
TCP. The average congestion window W in MSS-sized segments is given
as a function of the packet drop rate P.
Given a constant decrease b(w) of 1/2, this would give an increase
a(w) of w/Low_Window, or equivalently, a constant increase of
1/Low_Window packets per acknowledgement, for w > Low_Window.
Another possibility is Scalable TCP [K03], which uses a fixed
decrease b(w) of 1/8 and a fixed increase per acknowledgement of
0.01. This gives an increase a(w) per window of 0.005 w, for a TCP
with delayed acknowledgements, for pure MIMD.
The relative fairness between the alternate Linear response function
and the standard TCP response function is illustrated below in Table
8.
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Packet Drop Rate P Fairness Aggregate Window Bandwidth
------------------ -------- ---------------- ---------
10^-2 1.0 24 2.8 Mbps
10^-3 1.0 76 9.1 Mbps
10^-4 3.2 500 60.0 Mbps
10^-5 15.1 4179 501.4 Mbps
10^-6 31.6 39200 4.7 Gbps
10^-7 100.1 383795 46.0 Gbps
Table 8: Relative Fairness between the Linear HighSpeed and Standard
Response Functions.
One attraction of the linear response function is that it is scale-
invariant, with a fixed increase in the congestion window per
acknowledgement, and a fixed number of round-trip times between loss
events. My own assumption would be that having a fixed length for
the congestion epoch in round-trip times, regardless of the packet
drop rate, would be a poor fit for an imprecise and imperfect world
with routers with a range of queue management mechanisms, such as
the Drop-Tail queue management that is common today. For example, a
response function with a fixed length for the congestion epoch in
round-trip times might give less clearly-differentiated feedback in
an environment with steady-state background losses at fixed
intervals for all flows (as might occur with a wireless link with
occasional short error bursts, giving losses for all flows every N
seconds regardless of their sending rate).
While it is not a goal to have perfect fairness in an environment
with synchronized losses, it would be good to have moderately
acceptable performance in this regime. This goal might argue
against a response function with a constant number of round-trip
times between congestion events. However, this is a question that
could clearly use additional research and investigation. In
addition, flows with different round-trip times would have different
time durations for congestion epochs even in the model with a linear
response function.
The third column of Table 8, the Aggregate Window, gives the
aggregate congestion window of two competing TCP connections, one
with Linear HighSpeed TCP and one with Standard TCP, given the
packet drop rate specified in the first column. From Table 8, a
Linear HighSpeed TCP connection would receive fifteen times the
bandwidth of a Standard TCP in an environment with a packet drop
rate of 10^-5. This would occur when the two flows sharing a single
pipe achieved an aggregate window of 4179 packets. Given a round-
trip time of 100 ms and a packet size of 1500 bytes, this would
occur with an available bandwidth for the two competing flows of 501
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Mbps. Thus, because the Linear HighSpeed TCP is more aggressive
than the HighSpeed TCP proposed above, it also is less fair when
competing with Standard TCP in a high-bandwidth environment.
9. Tradeoffs for Choosing Congestion Control Parameters.
A range of metrics can be used for evaluating choices for congestion
control parameters for HighSpeed TCP. My assumption in this section
is that for a response function of the form w = c/p^d, for constant
c and exponent d, the only response functions that would be
considered are response functions with 1/2 <= d <= 1. The two ends
of this spectrum are represented by current TCP, with d = 1/2, and
by the linear response function described in Section 8 above, with d
= 1. HighSpeed TCP lies somewhere in the middle of the spectrum,
with d = 0.835.
Response functions with exponents less than 1/2 can be eliminated
from consideration because they would be even worse than standard
TCP in accomodating connections with high congestion windows.
9.1. The Number of Round-Trip Times between Loss Events.
Response functions with exponents greater than 1 can be eliminated
from consideration because for these response functions, the number
of round-trip times between loss events decreases as congestion
decreases. For a response function of w = c/p^d, with one loss
event or congestion event every 1/p packets, the number of round-
trip times between loss events is w^((1/d)-1)/c^(1/d). Thus, for
standard TCP the number of round-trip times between loss events is
linear in w. In contrast, one attraction of the linear response
function, as described in Section 8 above, is that it is scale-
invariant, in terms of a fixed increase in the congestion window per
acknowledgement, and a fixed number of round-trip times between loss
events.
However, for a response function with d > 1, the number of round-
trip times between loss events would be proportional to w^((1/d)-1),
for a negative exponent ((1/d)-1), setting smaller as w increases.
This would seem undesirable.
9.2. The Number of Packet Drops per Loss Event, with Drop-Tail.
A TCP connection increases its sending rate by a(w) packets per
round-trip time, and in a Drop-Tail environment, this is likely to
result in a(w) dropped packets during a single loss event. One
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attraction of standard TCP is that it has a fixed increase per
round-trip time of one packet, minimizing the number of packets that
would be dropped in a Drop-Tail environment. For an environment
with some form of Active Queue Management, and in particular for an
environment that uses ECN, the number of packets dropped in a single
congestion event would not be a problem. However, even in these
environments, larger increases in the sending rate per round-trip
time result in larger stresses on the ability of the queues in the
router to absorb the fluctuations.
HighSpeed TCP plays a middle ground between the metrics of a
moderate number of round-trip times between loss events, and a
moderate increase in the sending rate per round-trip time. As shown
in Appendix B, for a congestion window of 83,000 packets, HighSpeed
TCP increases its sending rate by 70 packets per round-trip time,
resulting in at most 70 packet drops when the buffer overflows in a
Drop-Tail environment. This increased aggressiveness is the price
paid by HighSpeed TCP for its increased scalability. A large number
of packets dropped per congestion event could result in synchronized
drops from multiple flows, with a possible loss of throughput as a
result.
Scalable TCP has an increase a(w) of 0.005 w packets per round-trip
time. For a congestion window of 83,000 packets, this gives an
increase of 415 packets per round-trip time, resulting in roughly
415 packet drops per congestion event in a Drop-Tail environment.
Thus, HighSpeed TCP and its variants place increased demands on
queue management in routers, relative to Standard TCP. (This is
rather similar to the increased demands on queue management that
would result from using N parallel TCP connections instead of a
single Standard TCP connection.)
10. Related Issues10.1. Slow-Start.
An companion internet-draft on "Limited Slow-Start for TCP with
Large Congestion Windows" [F02b] proposes a modification to TCP's
slow-start procedure that can significantly improve the performance
of TCP connections slow-starting up to large congestion windows.
For TCP connections that are able to use congestion windows of
thousands (or tens of thousands) of MSS-sized segments (for MSS the
sender's MAXIMUM SEGMENT SIZE), the current slow-start procedure can
result in increasing the congestion window by thousands of segments
in a single round-trip time. Such an increase can easily result in
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thousands of packets being dropped in one round-trip time. This is
often counter-productive for the TCP flow itself, and is also hard
on the rest of the traffic sharing the congested link.
[F02b] proposes Limited Slow-Start, limiting the number of segments
by which the congestion window is increased for one window of data
during slow-start, in order to improve performance for TCP
connections with large congestion windows. We have separated out
Limited Slow-Start to a separate draft because it can be used both
with Standard or with HighSpeed TCP.
Limited Slow-Start is illustrated in the NS simulator, for snapshots
after May 1, 2002, in the tests "./test-all-tcpHighspeed tcp1A" and
"./test-all-tcpHighspeed tcpHighspeed1" in the subdirectory
"tcl/lib".
In order for best-effort flows to safely start-up faster than slow-
start, e.g., in future high-bandwidth networks, we believe that it
would be necessary for the flow to have explicit feedback from the
routers along the path. There are a number of proposals for this,
ranging from a minimal proposal for an IP option that allows TCP SYN
packets to collect information from routers along the path about the
allowed initial sending rate [J02], to proposals with more power
that require more fine-tuned and continuous feedback from routers.
These proposals all are somewhat longer-term proposals than the
HighSpeed TCP proposal in this document, requiring longer lead times
and more coordination for deployment, and will be discussed in later
documents.
10.2. Limiting burstiness on short time scales.
Because the congestion window achieved by a HighSpeed TCP connection
could be quite large, there is a possibility for the sender to send
a large burst of packets in response to a single acknowledgement.
This could happen, for example, when there is congestion or
reordering on the reverse path, and the sender receives an
acknowledgement acknowledging hundreds or thousands of new packets.
Such a burst would also result if the application was idle for a
short period of time less than a round-trip time, and then suddenly
had lots of data available to send. In this case, it would be
useful for the HighSpeed TCP connection to have some method for
limiting bursts.
We do not in this document specify TCP mechanisms for reducing the
short-term burstiness. One possible mechanism is to use some form
of rate-based pacing, and another possibility is to use maxburst,
which limits the number of packets that are sent in response to a
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single acknowledgement. We would caution, however, against a
permanent reduction in the congestion window as a mechanism for
limiting short-term bursts. Such a mechanism has been deployed in
some TCP stacks, and our view would be that using permanent
reductions of the congestion window to reduce transient bursts would
be a bad idea [Fl03].
10.3. Other limitations on window size.
The TCP header uses a 16-bit field to report the receive window size
to the sender. Unmodified, this allows a window size of at most
2**16 = 65K bytes. With window scaling, the maximum window size is
2**30 = 1073M bytes [RFC 1323]. Given 1500-byte packets, this
allows a window of up to 715,000 packets.
10.4. Implementation issues.
One implementation issue that has been raised with HighSpeed TCP is
that with congestion windows of 4MB or more, the handling of
successive SACK packets after a packet is dropped becomes very time-
consuming at the TCP sender [S03]. Tom Kelly's Scalable TCP
includes a "SACK Fast Path" patch that addresses this problem.
The issues addressed in the Web100 project, the Net100 project, and
related projects about the tuning necessary to achieve high
bandwidth data rates with TCP apply to HighSpeed TCP as well
[Net100, Web100].
11. Deployment issues.11.1. Deployment issues of HighSpeed TCP
We do not claim that the HighSpeed TCP modification to TCP described
in this paper is an optimal transport protocol for high-bandwidth
environments. Based on our experiences with HighSpeed TCP in the NS
simulator [NS], on simulation studies [SA03], and on experimental
reports [ABLLS03,D02,CC03,F03], we believe that HighSpeed TCP
improves the performance of TCP in high-bandwidth environments, and
we are documenting it for the benefit of the IETF community. We
encourage the use of HighSpeed TCP, and of its underlying response
function, and we further encourage feedback about operational
experiences with this or related modifications.
We note that in environments typical of much of the current
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Internet, HighSpeed TCP behaves exactly as does Standard TCP today.
This is the case any time the congestion window is less than 38
segments.
Bandwidth Avg Cwnd w (pkts) Increase a(w) Decrease b(w)
--------- ----------------- ------------- -------------
1.5 Mbps 12.5 1 0.50
10 Mbps 83 1 0.50
100 Mbps 833 6 0.35
1 Gbps 8333 26 0.22
10 Gbps 83333 70 0.10
Table 9: Performance of a HighSpeed TCP connection.
To help calibrate, Table 9 considers a TCP connection with 1500-byte
packets, an RTT of 100 ms (including average queueing delay), and no
competing traffic, and shows the average congestion window if that
TCP connection had a pipe all to itself and fully used the link
bandwidth, for a range of bandwidths for the pipe. This assumes
that the TCP connection would use Table 12 in determining its
increase and decrease parameters. The first column of Table 9 gives
the bandwidth, and the second column gives the average congestion
window w needed to utilize that bandwidth. The third column show
the increase a(w) in segments per RTT for window w. The fourth
column show the decrease b(w) for that window w (where the TCP
sender decreases the congestion window from w to w(1-b(w)) segments
after a loss event). We note that the actual congestion window when
a loss occurs is likely to be greater than the average congestion
window w in column 2, so the decrease parameter used could be
slightly smaller than the one given in column 4 of Table 9.
Table 9 shows that a HighSpeed TCP over a 10 Mbps link behaves
exactly the same as a Standard TCP connection, even in the absence
of competing traffic. One can think of the congestion window
staying generally in the range of 55 to 110 segments, with the
HighSpeed TCP behavior being exactly the same as the behavior of
Standard TCP. (If the congestion window is ever 128 segments or
more, then the HighSpeed TCP increases by two segments per RTT
instead of by one, and uses a decrease parameter of 0.44 instead of
0.50.)
Table 9 shows that for a HighSpeed TCP connection over a 100 Mbps
link, with no competing traffic, HighSpeed TCP behaves roughly as
aggressively as six parallel TCP connections, increasing its
congestion window by roughly six segments per round-trip time, and
with a decrease parameter of roughly 1/3 (corresponding to
decreasing down to 2/3-rds of its old congestion window, rather than
to half, in response to a loss event).
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For a Standard TCP connection in this environment, the congestion
window could be thought of as varying generally in the range of 550
to 1100 segments, with an average packet drop rate of 2.2 * 10^-6
(corresponding to a bit error rate of 1.8 * 10^-10), or
equivalently, roughly 55 seconds between congestion events. While a
Standard TCP connection could sustain such a low packet drop rate in
a carefully controlled environment with minimal competing traffic,
we would contend that in an uncontrolled best-effort environment
with even a small amount of competing traffic, the occasional
congestion events from smaller competing flows could easily be
sufficient to prevent a Standard TCP flow with no lower-speed
bottlenecks from fully utilizing the available bandwidth of the
underutilized 100 Mbps link.
That is, we would content that in the environment of 100 Mbps links
with a significant amount of available bandwidth, Standard TCP would
sometimes be unable to fully utilize the link bandwidth, and that
HighSpeed TCP would be an improvement in this regard. We would
further contend that in this environment, the behavior of HighSpeed
TCP is sufficiently close to that of Standard TCP that HighSpeed TCP
would be safe to deploy in the current Internet.
We do not believe that the deployment of HighSpeed TCP would serve
as a block to the possible deployment of alternate experimental
protocols for high-speed congestion control, such as Scalable TCP,
XCP [KHR02], or FAST TCP [JWL03]. In particular, we don't expect
HighSpeed TCP to interact any more poorly with alternative
experimental proposals that would the N parallel TCP connections
commonly used today in the absence of HighSpeed TCP.
11.2. Deployment issues of Scalable TCP
We believe that Scalable TCP and HighSpeed TCP have sufficiently
similar response functions that they could easily coexist in the
Internet. However, we have not investigated Scalable TCP
sufficiently to be able to claim, in this document, that Scalable
TCP is safe for a widespread deployment in the current Internet.
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Bandwidth Avg Cwnd w (pkts) Increase a(w) Decrease b(w)
--------- ----------------- ------------- -------------
1.5 Mbps 12.5 1 0.50
10 Mbps 83 0.4 0.125
100 Mbps 833 4.1 0.125
1 Gbps 8333 41.6 0.125
10 Gbps 83333 416.5 0.125
Table 10: Performance of a Scalable TCP connection.
Table 10 shows the performance of a Scalable TCP connection with
1500-byte packets, an RTT of 100 ms (including average queueing
delay), and no competing traffic. The TCP connection is assumed to
use delayed acknowledgements. The first column of Table 10 gives
the bandwidth, the second column gives the average congestion window
needed to utilize that bandwidth, and the third and fourth columns
give the increase and decrease parameters.
Note that even in an environment with a 10 Mbps link, Scalable TCP's
behavior is considerably different from that of Standard TCP. The
increase parameter is smaller than that of Standard TCP, and the
decrease is smaller also, 1/8-th instead of 1/2. That is, for 10
Mbps links, Scalable TCP increases less aggressively than Standard
TCP or HighSpeed TCP, but decreases less aggressively as well.
In an environment with a 100 Mbps link, Scalable TCP has an increase
parameter of roughly four segments per round-trip time, with the
same decrease parameter of 1/8-th. A comparison of Tables 9 and 10
shows that for this scenario of 100 Mbps links, HighSpeed TCP
increases more aggressively than Scalable TCP.
Next we consider the relative fairness between Standard TCP,
HighSpeed TCP and Scalable TCP. The relative fairness between
HighSpeed TCP and Standard TCP was shown in Table 5 earlier in this
document, and the relative fairness between Scalable TCP and
Standard TCP was shown in Table 8. Following the approach in
Section 6, for a given packet drop rate p, for p < 10^-3, we can
estimate the relative fairness between Scalable and HighSpeed TCP as
W_Scalable/W_HighSpeed. This relative fairness is shown in Table 11
below. The bandwidth in the last column of Table 11 is the
aggregate bandwidth of the two competing flows given 100 ms round-
trip times and 1500-byte packets.
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Packet Drop Rate P Fairness Aggregate Window Bandwidth
------------------ -------- ---------------- ---------
10^-2 1.0 24 2.8 Mbps
10^-3 1.0 76 9.1 Mbps
10^-4 1.4 643 77.1 Mbps
10^-5 2.1 5595 671.4 Mbps
10^-6 3.1 50279 6.0 Gbps
10^-7 4.5 463981 55.7 Gbps
Table 11: Relative Fairness between the Scalable and HighSpeed
Response Functions.
The second row of Table 11 shows that for a Scalable TCP and a
HighSpeed TCP flow competing in an environment with 100 ms RTTs and
a 10 Mbps pipe, the two flows would receive essentially the same
bandwidth. The next row shows that for a Scalable TCP and a
HighSpeed TCP flow competing in an environment with 100 ms RTTs and
a 100 Mbps pipe, the Scalable TCP flow would receive roughly 50%
more bandwidth than would HighSpeed TCP. Table 11 shows the
relative fairness in higher-bandwidth environments as well. This
relative fairness seems sufficient that there should be no problems
with Scalable TCP and HighSpeed TCP coexisting in the same
environment as Experimental variants of TCP.
We note that one question that requires more investigation with
Scalable TCP is that of convergence to fairness in environments with
Drop-Tail queue management.
12. Related Work in HighSpeed TCP.
HighSpeed TCP has been separately investigated in simulations by
Sylvia Ratnasamy and by Evandro de Souza [SA03]. The simulations in
[SA03] verify the fairness properties of HighSpeed TCP when sharing
a link with Standard TCP.
These simulations explore the relative fairness of HighSpeed TCP
flows when competing with Standard TCP. The simulation environment
includes background forward and reverse-path TCP traffic limited by
the TCP receive window, along with a small amount of forward and
reverse-path traffic from the web traffic generator. Most of the
simulations so far explore performance on a simple dumbbell topology
with a 1 Gbps link with a propagation delay of 50 ms. Simulations
have been run with Adaptive RED and with DropTail queue management.
The simulations in [SA03] explore performance with a varying number
of competing flows, with the competing traffic being all standard
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TCP; all HighSpeed TCP; or a mix of standard and HighSpeed TCP. For
the simulations in [SA03] with RED queue management, the relative
fairness between standard and HighSpeed TCP is consistent with the
relative fairness predicted in Table 5. For the simulations with
Drop Tail queues, the relative fairness is more skewed, with the
HighSpeed TCP flows receiving an even larger share of the link
bandwidth. This is not surprising; with Active Queue Management at
the congested link, the fraction of packet drops received by each
flow should be roughly proportional to that flow's share of the link
bandwidth, while this property no longer holds with Drop Tail queue
management. We also note that relative fairness in simulations with
Drop Tail queue management can sometimes depend on small details of
the simulation scenario, and that Drop Tail simulations need special
care to avoid phase effects [F92].
[SA03] explores the bandwidth `stolen' by HighSpeed TCP from
standard TCP by exploring the fraction of the link bandwidth N
standard TCP flows receive when competing against N other standard
TCP flows, and comparing this to the fraction of the link bandwidth
the N standard TCP flows receive when competing against N HighSpeed
TCP flows. For the 1 Gbps simulation scenarios dominated by long-
lived traffic, a small number of standard TCP flows are able to
achieve high link utilization, and the HighSpeed TCP flows can be
viewed as stealing bandwidth from the competing standard TCP flows,
as predicted in Section 6 on the Fairness Implications of the
HighSpeed Response Function. However, [SA03] shows that when even a
small fraction of the link bandwidth is used by more bursty, short
TCP connections, the standard TCP flows are unable to achieve high
link utilization, and the HighSpeed TCP flows in this case are not
`stealing' bandwidth from the standard TCP flows, but instead are
using bandwidth that otherwise would not be utilized.
The conclusions of [SA03] are that "HighSpeed TCP behaved as forseen
by its response function, and appears to be a real and viable option
for use on high-speed wide area TCP connections."
Future work that could be explored in more detail includes
convergence times after new flows start-up; recovery time after a
transient outage; the response to sudden severe congestion, and
investigations of the potential for oscillations. We invite
contributions from others in this work.
13. Relationship to other Work.
Our assumption is that HighSpeed TCP will be used with the TCP SACK
option, and also with the increased Initial Window of three or four
segments, as allowed by [RFC3390]. For paths that have substantial
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reordering, TCP performance would be greatly improved by some of the
mechanisms still in the research stages for robust performance in
the presence of reordered packets.
Our view is that HighSpeed TCP is largely orthogonal to proposals
for higher PMTU (Path MTU) values [M02]. Unlike changes to the
PMTU, HighSpeed TCP does not require any changes in the network or
at the TCP receiver, and works well in the current Internet. Our
assumption is that HighSpeed TCP would be useful even with larger
values for the PMTU. Unlike the current congestion window, the PMTU
gives no information about the bandwidth-delay product available to
that particular flow.
A related approach is that of a virtual MTU, where the actual MTU of
the path might be limited [VMSS,S02]. The virtual MTU approach has
not been fully investigated, and we do not explore the virtual MTU
approach further in this document.
14. Conclusions.
This document has proposed HighSpeed TCP, a modification to TCP's
congestion control mechanism for use with TCP connections with large
congestion windows. We have explored this proposal in simulations,
and others have explored HighSpeed TCP with experiments, and we
believe HighSpeed TCP to be safe to deploy on the current Internet.
We would welcome additional analysis, simulations, and particularly,
experimentation. More information on simuations and experiments is
available from the HighSpeed TCP Web Page [HSTCP]. There are
several independent implementations of HighSpeed TCP [D02,F03] and
of Scalable TCP [K03] for further investigation.
We are bringing this proposal to the IETF to be considered as an
Experimental RFC.
15. Acknowledgements
The HighSpeed TCP proposal is from joint work with Sylvia Ratnasamy
and Scott Shenker (and was initiated by Scott Shenker). Additional
investigations of HighSpeed TCP were joint work with Evandro de
Souza and Deb Agarwal. We thank Tom Dunigan for the implementation
in the Linux 2.4.16 Web100 kernel, and for resulting experimentation
with HighSpeed TCP. We are grateful to the End-to-End Research
Group, the members of the Transport Area Working Group, and to
members of the IPAM program in Large Scale Communication Networks
for feedback. We thank Glenn Vinnicombe for framing the Linear
response function in the parameters of HighSpeed TCP. We are also
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